Our model of the atom has the electrons on the outer boundary and in this location they will be the particles that interacts with other substances in chemical reactions. But we also know from the groups on the periodic table (alkali metals, alkaline earth metals, halogens, and noble gases) that the same reactivity and properties reoccur as more electrons are added. This means that adding electrons (recall electrons equal the protons in a neutral atom) does not gradually change the properties of atoms, but rather properties of atoms repeat as electrons increase. Therefore, the arrangement of electrons changes across the periodic table (to change properties) and then the arrangement returns to something similar so that the properties of elements repeat in the families of elements. Thus, the origin of an elements chemistry must lie in the arrangement of electrons.
The Bohr Model of Electron Configuration
The arrangement of electrons in the atom also needs a theory that predicts and explains the properties. Many of the early theories suggested that the electrons traveled like planets in orbits about the nucleus. The problem with this theory was that it was known that an object traveling around an attractive center will slowly move towards the center. So if the orbit theory was true then some atoms would be spontaneously changing as electrons and protons reacted when the electron spiraled into the attractive, positive nucleus.
Below is a brief description of the quantum model of the atom. You are encouraged to look at these two sources for a complete description: Physics 2000 at http://www.colorado.edu/physics/2000/atomic_lab.html
and Hyperphysics at http://hyperphysics.phy-astr.gsu.edu/hbase/bohrcn.html#c1
Neils Bohr suggested that electrons had to remain in specific energy levels that had fixed energies. This restriction forced electrons to maintain their energy. In support of this theory, he showed calculations of the emission spectra of hydrogen gas that matched his theory. Emission spectra is the release of light after energy has been added to a substance. Unfortunately, this was the only substance that Bohr’s model fit. Fortunately, Bohr was able to put together a number of ideas and theories and point the way to the model we use today.
Bohr Model and Explanation- the mathematics of Waves and Light
The mathematics needed for a deep understanding of quantum theory is quite complex. Here are two simple, basic equations that show the relationship between the energy of light emitted (or absorbed) by an atom and the energy difference between energy levels that the electrons travel in. Different types of light have different frequencies, ?. When you are changing radio channels you are changing frequencies that are emitted from each radio stations (light is all parts of the electromagnetic spectra which includes radio waves, microwaves, infrared waves, visible light, ultraviolet light, x-rays, and gamma rays). Another measurement of light is wavelength, ?. Frequency and wavelength are related to each other by the speed of light, c: c = ?•? (the speed of light is a constant through a vacuum, 3 X 108 m/s).
Energy of any frequency of light is calculated by using another constant, h. h is plank’s constant and equals 6.63 X 10–34 m2kg/s. Plank’s constant is the smallest step of energy that can separate energy levels in an atom or between frequencies of light (on a normal scale energy is continuous because Plank’s constant is extremely small and differences in the steps are unobservable). Thus E = h? and has units of measurement of joules, J.
Hydrogen Emission Spectrum (Visible Light, Balmer Series)
These two equations were used by Bohr to support his model of the arrangement of electrons for the hydrogen electron. Energy is emitted as light when an electron loses energy to move to a lower level. But the change has to take place in steps of h, Plank’s constant. If the change was gradual or varied, then the bands of light would broaden and create a rainbow of light. Since the change has to be in steps, the lights appear as distinct lines. Each color is related to a wavelength, ?, or a frequency, ?. The frequency and wavelength of light is related to the energy, E = ?•h, needed to change energy levels. Comparing the observed light spectra with the energy predicted by theory, provides evidence for the correctness of the theory.
Schödinger and others showed that the behavior or electrons follows the pattern of waves better than as objects that obeying traditional physics like the movement of planets, which is what Bohr’s model was based on. They modified Bohr’s model using wave equations, or functions, to model the energy of electrons. The result is the quantum model, the Schödinger model, of the atom that correctly predicts the behavior of electrons and provides us with a physical representation of the location of electrons that we call electron configuration (electrons in compounds can also be modeled by quantum theory, but such models are much more complex).